1. Field of the Invention
The invention relates to an optically non-linear active waveguiding material comprising an optically transparent polymer and an optically non-linear active dopant ,comprising a donor-.pi.-acceptor unit.
2. Description of the Prior Art
In optically non-linear materials, under the influence of an external field of force (such as an electric field of force E), non-linear polarization occurs. In the case of organic molecules this is called an induced dipole moment. The induced dipole moment (.mu..sub.ind) may be represented as follows: EQU .mu..sub.ind =.alpha.E+.beta.EE+.gamma.EEE+ [1]
wherein .alpha. stands for the linear hyperpolarizability, .beta. represents the (non-linear) hyperpolarizability, .gamma. stands for the second hyperpolarizability, etc.
Non-linear electric polarisation (.beta. and .gamma. do not equal zero) may give rise to a number of optically non-linear phenomena, such as frequency doubling and the Pockels effect. By utilizing these phenomena it is possible to employ this material in optically active waveguiding structures such as optical switches, frequency doublers, etc.
To this end the organic optically non-linear active material is to be applied to a substrate in the form of thin layers. Three polymer systems are suitable for this purpose: guest-host polymer systems, functionalized polymers, and polymer networks. In the first system, which is by far the easiest to prepare, an optically transparent polymer contains optically non-linear active compounds as dopants.
Such an optically non-linear active waveguiding material is disclosed in GB 2 189 624, where an optically transparent polymer is impregnated with an optically non-linear dopant comprising a donor-.pi.-acceptor unit.
Most organic optically non-linear active compounds owe their nonlinear optical properties to so-called donor-.pi.-acceptor units. By this term are meant, groups composed of an electron-donating group and an electron-accepting group coupled to the same conjugated .pi.-system. Compounds containing such a material were found to have a comparatively high hyperpolarizability (.beta.).
For several years now efforts have been made in industry to prepare materials of a higher hyperpolarizability, for instance by expanding the donor-.pi.-acceptor units. While the hyperpolarizability is thus increased, there is, simultaneously, a shift in the charge-transfer absorption band to a longer wavelength. In consequence, this material has limited applicability in the case of, say, frequency doubling. For, it is inadvisable for the optically non-linear material to have absorption bands in the very working range in which frequency doubling is to be carried out: ordinarily, electromagnetic radiation having a wavelength of 700 to 1300 nanometers (nm) is passed through a frequency doubler by means of a laser, which results in a light source emitting a wavelength of half that length, i.e., in the range of approximately 350 to 650 nm. Preparing an optically non-linear active material without any absorption bands in the 350 to 650 nm range has proved to be a difficult affair.
A drawback to the conventional guest-host systems is that the stability of the poled films frequently leaves much to be desired. The effective non-linear coefficients (d.sub.33) of the poled film were found to decrease after some time. The effective coefficient is indicative of the non-linear optical behavior of the film. It takes account of the hyperpolarizability of each optically non-linear active dopant molecule and the degree to which it contributes within the film to the overall non-linear optical behaviour. In addition, the solubility of the dopant in the optically transparent polymer frequently causes problems. In general, only up to 5 weight percent of dopant can be incorporated into the already known guest-host systems.